atoomfractie

Atomfractie is the fraction of atoms of a given isotope within a sample of an element. It describes how many atoms of that isotope are present relative to the total number of atoms of all isotopes of the same element.

Mathematically, for an element with isotopes i, j, etc., the atom fraction a_i is defined as a_i = N_i / sum_j N_j, where N_i is the number of atoms of isotope i and the sum runs over all isotopes of the element. In the common two-isotope case, a_i + a_j = 1, and the isotopic ratio R = N_i / N_j relates to the fractions by a_i = R / (1 + R) and a_j = 1 / (1 + R).

Atom fractions are central to fields such as geochemistry, cosmochemistry, environmental science and various branches of chemistry, where they are measured by mass spectrometry to determine the isotopic composition of elements like carbon, hydrogen, nitrogen, oxygen and sulfur. They are often reported as natural abundances or as deviations from a standard (delta notation) in order to study processes that fractionate isotopes.

Typical natural abundances example: for carbon, ^12C has a fraction of about 0.989 and ^13C about 0.011. For oxygen, ^16O is about 0.997, ^17O about 0.00038, and ^18O about 0.00205. Variations in atom fractions arise from physical, chemical, or biological processes and can be used to trace sources and histories of materials.

In practice, atom fraction and mole fraction are numerically equivalent when describing isotopic compositions, since both are based on the same counts of atoms.